Because the cable television (CATV) network shares frequencies with licensed radio frequency (RF) services, care must be taken to prevent signal leakage from the cable plant degrading licensed services, and to prevent signal ingress from licensed services degrading signals on the cable plant.
Ingress is most problematic in the 5-43 MHz spectrum used for upstream (return path) transmission, i.e. from the customer to the head end. Common sources of ingress signals include amateur radio, remote controls for RC toys, AM broadcast, shortwave broadcast, and transients from consumer electronics.
On the other hand, the Federal Communications Commission (FCC) requires that leakage from the CATV plant be tightly controlled to minimize interference to other services, particularly the aviation band around 110-140 MHz.
Currently, specialized scanning spectrum analyzer equipment is used at the head end or node to monitor the return path of each node for the presence and strength of ingress or interference and to create or store alarms when interference takes place. When a sufficiently harmful interferer is detected, personnel are dispatched to identify and address the physical point of entry of the interference, be it a loose connector, broken shield, home wiring, etc. Existing monitoring equipment identifies only the node where ingress is occurring; the physical point of ingress is pinpointed by systematically segmenting the network to isolate the source.
Ingress troubleshooting is performed manually by repeatedly segmenting the network. For instance, a time-based technique is used by Arcom's Hunter to find control point discovery (CPD), based (presumably) on cross-correlation between the downstream and upstream channels, but there does not appear to be any timing-based ingress detection or transmit-based ingress technology in the market to date.
For detecting signal leakage out of the CATV plant radiating into free space, signal-strength detection techniques are currently used, perhaps in combination with triangulation using a directional antenna element, to track the leakage to its source.
The radiating leaked signal strength can vary with time, rendering a series of measurements taken at various locations over extended time periods unreliable. Both directional antenna and signal strength measurements are vulnerable to interference from other signal sources in the same frequency range. Furthermore signal leakage from cable plant may not occur uniformly in all directions, rendering the signal strength measurements difficult to interpret, or even totally unusable in the worst case.
Some leakage detection equipment contains GPS modules, but they are used for meter positioning only. Talbot et al. (US Patent Application 20040012520) describes the use of the highly precise cesium-rubidium clocks in orbiting navigation satellite systems such as the global positioning system (GPS) as standards for calibrating electronic distance meters (EDM). Baseline measurements made by the EDM are therefore not subject to mis-calibrations and drift as long as the satellite navigation receiver is locked onto and tracking the navigation satellites.
So, while the locations where leakage detection measurements are taken can be determined with adequate accuracy, none of the prior art techniques attempt to identify the leaked signal by its content or otherwise exclude from consideration extraneous signal sources.
An object of instant invention is to provide a method for locating cable plant faults using the characteristics of one or more signals in conjunction with their total propagation time through cable plant and free-space.